1. Field of the Invention
The present invention relates to an optical semiconductor device for use in optical communication and transmission techniques and the like, and in particular, to a member holding an optical transmission line and an optical module both of which have an improved wiring part.
2. Description of the Related Art
A technique has recently been researched and developed which uses a coupling scheme called direct optical coupling (i.e., butt joint) to reduce an implementation cost; this technique arranges an optical semiconductor device such as a light emitting element or a an optical sensing element and an optical fiber opposite each other and immediately close to each other to achieve optical coupling without using any lenses.
With the direct optical coupling, a light beam emitted from the semiconductor device or optical fiber is transmitted through a medium and diverged in the medium, the medium having a substantially equal refractive index and having no waveguide mechanisms, for example, air or a refractive index matching material, unless the medium is provided with any lens effect. This increases the light rays of the beam, which are incident in area or areas other than the wave-guiding portion (core) of the optical fiber or an active area (optical sensing area) of the optical sensing element. This in turn reduces light coupling efficiency and thus noise resistance, while increasing the amount of stray light rays. As a result, the amount of another noise (for example, crosstalk noise) increases to affect signal transmissions.
It is thus important to arrange the optical semiconductor device and the optical fiber as close to each other as possible to prevent light rays from reaching unwanted area or areas. For example, a light beam emitted from a multimode optical fiber with a numerical aperture (NA) of 0.21 and a core diameter of 50 μm is diverged in the air at a spread angle of about 12°. The optical semiconductor device and the optical fiber thus need to be arranged closer to each other so that the distance between them is about several tens of μm. Moreover, standard optical fibers need to have a radius of curvature of about 30 mm and cannot be bent at a right angle. The optical fiber is consequently placed in a surface having an axial direction substantially parallel to a surface on which an optical module is mounted. The optical fiber in the optical module therefore needs to avoid projecting in a direction perpendicular to a substrate on which it is mounted, to reduce the thickness of the entire apparatus.
To meet these various requirements, Jpn. Pat. Appln. KOKAI Publication No. 2000-34072 proposes an optical module provided with a member holding an optical transmission line, which has a holding hole in which an optical fiber is inserted to hold the optical fiber. Thus, the optical fiber is held in the holding hole, and an optical semiconductor device is mounted on a surface of the member, on which the holding hole is open.
However, the optical module proposed in Jpn. Pat. Appln. KOKAI Publication No. 2000-34072 poses problems described below. In this conventional optical module, the member holding the optical transmission line has a surface in which an optical input port or an optical output port of the optical transmission line is located. An electrode is also provided on this surface and the optical semiconductor device is mounted on the electrode. In the optical module configured as described above, the member holding the optical transmission line has a thermal expansion coefficient different from that of the optical semiconductor device. Consequently, a heat cycle, a thermal impact, or the like concentrates stress on the electric connection between the member and the optical semiconductor device. Metal fatigue is thus likely to occur to destroy the module. Consequently, an underfill material (adhesive) consisting of resin or the like needs to be filled between the optical semiconductor device and the surface of the optical transmission line holding member on which the optical semiconductor device is mounted. This reduces the stress concentrated on the electric connection to improve the reliability of the module.
In this optical module, the optical input portion or the optical output portion of the optical semiconductor device is preferably placed in proximity to the optical input port or the optical output port of the optical transmission line. Further, the gap into which the underfill material is filled is very narrow; its width is set at, for example, 5 to 50 μm. The resin as the underfill material thus needs to be less viscous. After being completed, the optical module is electrically connected to an external mounting substrate or an IC. In this optical module, if the underfill material is less viscous, the resin may travel along the electrode to contaminate its connecting point. In particular, a module that needs to exhibit a high frequency characteristic is likely to be contaminated because it needs to have as small an electrode length as possible.